The present invention relates to the handling of ocean bottom seismic cable, more specific the invention relates to a device for the retrieval of the cable from the ocean floor.
Seismic investigations at sea are usually carried out by using a surface seismic investigating apparatus and method where a seismic cable with monitoring equipment is towed behind a vessel, where, for example, air is "shot" into the water and forms sound waves which are reflected by the layers in the underlying rock formation, and where these reflections are registered by means of the monitoring equipment of the seismic cable.
Computer analyses of the registered pattern of reflections provide a basis for mapping structures of interest below the ocean floor.
Ocean bottom seismic investigating apparatus and method bear a strong resemblance to surface seismic investigating apparatus and method, with the exception that the monitoring cables are positioned on the ocean floor. Colloquially, seismic investigating apparatus and method are referred to by the shortened term "seismic," which is used for that meaning in this document, so that the monitoring equipment is in direct contact with the floor. The advantage of ocean bottom seismic is that by direct contact between the underlying floor and the monitoring equipment the sensitivity of the equipment is increased, and the measurements obtained are more accurate and detailed, giving the basis for a even more detailed mapping of the underlying structures. However, ocean bottom seismic is more expensive to carry out than surface seismic.
Typically, an ocean bottom seismic cable is 10 to 12 km in length and is composed of a plurality of sections of about 300 m in length fitted together using a connecting piece, and where the monitoring equipment which is to record signals from the ocean floor is placed at defined positions on the cable.
The cable can be laid on the ocean floor by being released from the stem of a vessel travelling at a speed of 2 to 5 knots across the ocean floor, upon which the monitoring cable is to be placed. During the registration of ocean bottom seismic, a plurality of cables are laid in parallel relation and spaced apart at a given distance on the ocean floor. Normally, the work will be carried out continuously so that two to four cables are used for monitoring, while the monitoring field is moved across the ocean floor by taking up the cable which lies outermost in the monitoring field, and moving this cable parallel with the other cables across to the other side of the monitoring field. During an operation of this kind three vessels are normally involved, one vessel which "shoots" and two vessels which alternate between monitoring the laid cables and moving the monitoring field by taking up a cable along one side edge of the field and moving across to the other side edge thereof.
Today, the ocean bottom seismic cable is retrieved by running the cable over a wheel which is located on the side at the front of the vessel. The cable is led over a wheel and in between a powered rubber wheel which ensures that the cable is passed abaft on the boat in a groove to the stem, where a block, suspended in a travelling crane lays the cable out across the deck so as to facilitate the location of the specific read-off points and the connecting pieces. The cable thus lies like a "heap of spaghetti" on the deck, where only the read-off points and the connecting pieces are accessible for measurement. En route to and from the survey, there may be two to three such cables lying on top of one another on a deck of a vessel of this kind.
A major and costly problem in connection with such ocean bottom seismic is that the cable tends to incur some damage, and much time is spent repairing this. This repair work is usually carried out by locating the fault through measurements at the defined read-off points and then replacing one or more sections.
Studies have also shown that about 50% of the damage to the cable takes place on board the boat during retrieval and deployment. During the retrieval of the cable, the cable is damaged because of jerks and uneven pull on the cable during the retrieval thereof. Normally, the cable runs, as mentioned above, over a wheel at the front on the side of the vessel. This wheel is fixed and follows the motions of the vessel, so that in rough seas there will be jerks on the cable. Moreover, the cable must run straight up from the ocean bottom, so that there is no tension in any direction along the ocean bottom, as this could cause the cable to become caught on objects on the bottom.